Remote Keyless Entry Systems

Basic Description

Remote keyless entry systems are found on virtually all cars currently sold in the U.S.
These systems allow the holder of the key (or key fob) to unlock the driver's door, all
doors and/or the trunk of a car at the push of a button as long as the vehicle is within
about 50 feet. The key transmits an encoded RF signal to the main computer,
which actuates the door locks. Some remote keyless systems are also capable of performing
other functions such as arming or disarming an alarm, or even starting the car's engine.

In order to prevent unauthorized users from detecting and reusing the signals transmitted
by the key, a hopping/rolling code is used.
The controller chip and the key share an algorithm that generates a 40-bit code.
When the key is pressed, the
transmitter in the key transmits the 40 bit code along with the function code.
The function code tells the controller what action to take (e.g. unlock the driver's door).
Every time the transmitter button is pushed a new 40-bit code is generated using a
Pseudo-Random Number Generator. The same generator is used in the
receiver to generate the same 40-bit code. This way the transmitter and the receiver
remain synchronized. Since the transmitter's signal may not be picked up by the receiver
every time the button is pushed, the receiver also calculates the next 256 codes that the
transmitter is expected to transmit and accepts any of them. The transmitter and receiver
re-synchronize
themselves every time a code is accepted successfully. Since there are roughly a trillion
possible codes, the odds of a randomly selected code being one of the 256 acceptable codes
are extremely small.

Some remote keyless entry systems are two-way systems that allow the vehicle to
send status information back to the user. These systems often have a small screen on the fob that informs the user about fluid levels, tire pressure, windows status (up/down/broken), door status (open/closed),
diagnostic data, and lock status.

The most advanced remote keyless entry systems are typically named Advanced Keys or Smart Keys. These keys employ RFID technology and have passive functions in addition to their active functions performed by pressing a button. A vehicle with a smart key can sense when the key is within a certain range of the vehicle (typically 1.5 meters) and then will unlock the driver car door or wait until the driver touches a specific section of the door. These keys typically also include passive ignition capabilities. If the driver is sitting in the driver's seat and pushes the ignition button, the car will turn on only when the smart key is within range.

Today some remote entry systems allow the driver to use their smart phone to replace their key or key fob. One example of this is Viper's Smart Start system. In this case, an application on the phone outputs a signal to the vehicle's receiver through the cell phone network instead of the key or key fob. This allows the customer to use only one device to operate all of his or her vehicles.

Some newer smart key fobs provide drivers with remote access to vehicle information while helping to ensure the security of the data exchange. In some cases, this information can be accessed via a portable device such as a smart phone. Data that can be accessed and managed may include mileage, fuel level, tire pressure, maintenance warnings and alerts, personal comfort and preference settings, and vehicle location.